What leads to antiphospholipid syndrome?

It can be genetic. One can inherit this syndrome, or it may arise without any apparent history or cause. It is associated with multiple medical problems, including recurrent pregnancy loss.
It is autoimmune. Antiphospholipid syndrome (aps) is an autoimmune condition which can be its own entity or associated with other connective tissue diseases such as lupus. A diagnosis requires thorough evaluation by a rheumatologist. Typically, we don't make a diagnosis after one test result. The general rule of thumb is to repeat the test 3 months after the first one to confirm. There is more than one test.